Re: [Qemu-devel] [PATCH] linux-user: Add binfmt wrapper

[Riku Voipio <riku.voipio@iki.fi>]

On Tue, Jul 15, 2014 at 05:11:48PM +0200, Joakim Tjernlund wrote:
> Riku Voipio <riku.voipio@iki.fi> wrote on 2014/07/15 16:12:26:
> > On Mon, Jul 14, 2014 at 05:38:49PM +0200, Joakim Tjernlund wrote:
> > > Alexander Graf <agraf@suse.de> wrote on 2014/07/14 17:21:33:
> > > > On 14.07.14 16:38, Joakim Tjernlund wrote:
> > > > > The popular binfmt-wrapper patch adds an additional
> > > > > executable which mangle argv suitable for binfmt flag P.
> > > > > In a chroot you need the both (statically linked) qemu-$arch
> > > > > and qemu-$arch-binfmt-wrapper. This is sub optimal and a
> > > > > better approach is to recognize the -binfmt-wrapper extension
> > > > > within linux-user(qemu-$arch) and mangle argv there.
> > > > > This just produces on executable which can be either copied to
> > > > > the chroot or bind mounted with the appropriate -binfmt-wrapper
> > > > > suffix.
> > > > >
> > > > > Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
> > > > 
> > > > Please make sure to CC Riku on patches like this - he is the 
> linux-user 
> > > > maintainer.
> > > 
> > > Doesn't he read the devel list? Anyhow CC:ed
> > 
> > I do - but CC gets directly to my inbox while qemu-devel goes to an
> > folder.
> > 
> > I take from this discussion, that this patch has been superceded by the
> > Patch at: http://patchwork.ozlabs.org/patch/369770/ ?
> 
> BTW, any chance qemu binfmt could fixup the ps output from within a 
> container:
>  jocke-ppc2 ~ # ps uaxww
> USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
> root         1  0.1  0.0 4138016 7600 ?        Ss   17:02   0:00 
> /usr/bin/qemu-ppc /sbin/init /sbin/init
> root        79  0.0  0.0 4138016 5792 ?        Ss   17:02   0:00 
> /usr/bin/qemu-ppc /usr/sbin/sshd /usr/sbin/sshd
> root       293  0.0  0.0 4137952 4072 ?        Ss   17:02   0:00 
> /usr/bin/qemu-ppc /bin/busybox /bin/busybox udhcpc -x hostname:jocke-ppc2 
> --interface=eth0 --now --script=/lib/netifrc/sh/udhcpc-hook.sh 
> --pidfile=/var/run/udhcpc-eth0.pid
> root       334  0.3  0.0 4138016 5964 tty1     Ss+  17:02   0:00 
> /usr/bin/qemu-ppc /sbin/agetty /sbin/agetty 38400 tty1 linux
> root       335  3.1  0.0 4138048 7064 console  Ss   17:02   0:00 
> /usr/bin/qemu-ppc /bin/login /bin/login -- root
> root       336  3.3  0.0 4138016 9764 console  S    17:02   0:00 
> /usr/bin/qemu-ppc /bin/bash -bash
> root       340  0.0  0.0 4138016 6336 ?        R+   Jul10   0:00 /bin/ps 
> ps uaxww
> 
> As you can see, qemu-ppc is visible. 

This isn't something binfmt could do. ps uses /proc/$pid/exe to map the
right binary. However, qemu already fixes /proc/self/exe to point to
right file - as you can see from the last line where "ps uaxww" doesn't
have qemu shown. So it would be possible to make do_open() in syscall.c do
similar mapping for /proc/$pid/exe. 

Exactly how and when the qemu processes should be hidden within qemu
needs careful thought thou. A naive approach would check if
/proc/$pid/exe points to same binary as /proc/self/exe, hiding at least
same architecture qemu processes.

Riku